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Retrieving Oil and Recycling Surfactant in Surfactant-Enhanced Soil Washing Yanjie Xu, Yuandi Zhang, Xuefeng Liu, Hui Chen, and Yun Fang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04614 • Publication Date (Web): 20 Feb 2018 Downloaded from http://pubs.acs.org on February 20, 2018

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ACS Sustainable Chemistry & Engineering

Retrieving Oil and Recycling Surfactant in Surfactant-Enhanced Soil Washing Yanjie Xu, Yuandi Zhang, Xuefeng Liu,* Hui Chen and Yun Fang Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Lihu Ave. 1800, Wuxi 214122, PR China. * [email protected] ABSTRACT: How to simultaneously realize retrieving oil and recycling surfactant in the remediation of leaked oil-polluted soil by means of surfactant-enhanced soil washing is still a significant challenge. Here, we reported for the first time a novel CO2-switchable anionic surfactant, 11-dimethylamino-undecyl sulfate sodium salt (DUSNa), to retrieve leaked oil and recycle surfactant simultaneously in the process of oil-polluted sand remediation by DUSNaenhanced soil washing. Because a CO2-switchable tertiary amine group had been incorporated into a traditional anionic surfactant of sodium alkyl sulfate to form DUSNa, DUSNa was readily converted into its inactive form of DUS upon CO2 treatment, leading to complete oil/water phase separation from a DUSNa-stabilized oil-in-water emulsion. The formed DUS was insoluble in both the oil and the water, but was suspended or precipitated in the lower aqueous phase, allowing subsequent retrieval of almost surfactant-free oil. Recycling of DUSNa was enabled by its recovery from DUS by treatment with N2 or NaOH. Upon CO2 treatment, both around 92.1% of the oil and around 90.8 % of the DUSNa at least could be retrieved and recycled over three cycles of DUSNa-enhanced soil washing.

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KEYWORDS: Oil-polluted soil remediation, Sustainability, Resource recovery, Surfactantenhanced soil washing, Recyclable surfactant, Leaked oil retrieval

INTRODUCTION Soil contamination by petroleum-based oil is an extremely important and worldwide environmental problem.1–4 Remediation of oil-contaminated soil is of high priority worldwide.5–7 On the other hand, the magnitude of the issue of oil leak is large; for example, in the U.S. there are around 80,000 underground tanks currently leaking into the surrounding soil.4 Crude oil leaking into the environment worldwide is estimated to amount to around 0.4 billion tons annually.2 If the leaked oil could be retrieved and subsequently reused, it would be beneficial not only in terms of environmental remediation, but also for saving resources. Surfactant-enhanced soil washing (SESW) is one of the most versatile remediation techniques for virtually all target organic contaminants.1–5,8–10 Surfactants can reduce the oil–water interfacial tension (σ) and decrease the oil–soil attraction. SESW may be considered as a nondestructive remediation technique for the leaked oil. Unfortunately, surfactants tend to stabilize an emulsion of the oil in water (O/W emulsion) long after the SESW is performed, making it difficult to separate the oil from the water.

1–5,8–10

Thus, efficient methods for

demulsification of the obtained emulsions are desirable. Switchable or stimuli-responsive surfactants8,11 have the potential to solve this problem because of their reversible interconversions between active and inactive forms in response to stimuli,11 which makes such surfactant-stabilized emulsions are also switchable.12,13 CO2-switchable surfactants seem ideal for the SESW because CO2 treatment leaves no obvious by-products,14 but many such surfactants are cationic.8 Cationic surfactants do not perform well in SESW because of the adsorption.5,15 CO2-switchable anionic surfactants (CSASs) are, however, suitable for SESW.8,16 Elegant


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examples of CSASs such as carboxylates, benzoates, and phenolates have recently been reported.8 Unfortunately, efficient recycling CSASs from the liquid matrix is still challengeable.8 There is increasing awareness of the need for recovery and reusability of the surfactants for SESW, because of the potential negative effects of synthetic surfactants and the costeffectiveness of the process.1,9,10 Pioneering research revealed that the surfactant could be partially recovered after an appropriate degradation treatment, however, most of the target organic pollutants in the soil washing solution were destroyed.1,9,10 To facilitate demulsification and to retrieve the potentially reusable oil as well as the surfactant simultaneously, a CO2switchable anionic surfactant, which was a combination of dodecyl seleninic acid and N,N,N’,N’tetramethyl-1,2-ethylenediamine (DSA–DMEA) had been reported very recently by us.17 Unfortunately, DSA–DMEA did not work well in SESW. Therefore, novel CSASs are urgently required for use in SESW, in particular, those for which the inactive form is insoluble in both oil and water.

Scheme 1. Reversible switching between 11-dimethylamino-undecyl sulfate sodium (DUSNa) and its inactive form (DUS) under the stimuli of CO2, N2 and/or NaOH.


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In this study, we have employed a novel CSAS of 11-dimethylamino-undecyl sulfate sodium salt (DUSNa, Scheme 1) to remedy mineral oil (D80)-contaminated sand by SESW. DUSNa liberates the oil from the sand to give an O/W emulsion; the resulting O/W emulsion can be conveniently separated from the sand and rapidly demulsified by CO2 bubbling at room temperature to induce oil–water phase separation, allowing subsequent retrieval of almost surfactant-free oil. The DUSNa is converted into the inactive form DUS (Scheme 1). The formed DUS is insoluble in both the oil and the water, but is suspended or precipitated in the lower aqueous phase. Consequently, the oil and the suspension of DUS in water may be conveniently separated. The DUS may be restored to DUSNa by N2 bubbling at 90 oC or alkalization with NaOH at room temperature and then recycled. EXPERIMENTAL SECTION Materials. DUSNa was prepared according to Scheme S1 and its molecular structure was characterized by means of electrospray ionization mass spectrometry (ESI MS; see Figure S1) and 1H nuclear magnetic resonance spectrometry (1H NMR; see Figure S2), respectively. All other chemicals were commercial available and used as received. Methods. The interfacial tensions at the interfaces of air–aqueous sample (γ, mN m-1) and mineral oil–aqueous sample (σ, mN m-1) were measured by the drop volume method and spinning drop technology, respectively.13 A mixture of mineral oil D80 and aqueous surfactant solution with a volume ratio of 1:1 was homogenized to produce emulsion. The relative stability of the emulsion was measured as the time needed to separate 1 mL of H2O from 6 mL of emulsion (t1mL, min) at 30 °C.17 The type of emulsion was determined by the dilution method.18 Emulsion droplets were visualized using a Nikon-80i fluorescence microscope system (Nikon, Japan) and the emulsions were dyed with trace amount of Nile red in advance. The number


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distribution of diameters of oil droplets dispersed in aqueous solution was measured by a S3500Laser particle size analyser system (Microtrac, USA). For destabilization of emulsion, dry CO2 was bubbled at a rate of 60 mL min−1 for 15 min; for recovering the emulsion, dry N2 was bubbled at a rate of 60 mL min−1 for 15 h. The emulsion was also recovered by NaOH basification. Krafft temperature (TK, °C) was estimated from the solution temperature of DUSNa at 1 wt %.19,20 The SESW was carried out at room temperature using oil-contaminated sands.8 The chemical oxygen demand (COD) and total organic carbon (TOC) of samples were measured by a COD Analyzer (5B–1F, Lianhua Tech., China) and TOC Analyzer (VCPH, Shimadzu, Japan), respectively. Detailed information for the aforementioned and other experiments can be found in the Supporting Information.

Figure 1. Snapshots of the initial emulsion (a), the phase separation triggered by CO2 (b), the recovered emulsion by N2 bubbling (c) or NaOH (d). The initial emulsion was diluted with (e)


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0.1 mol L−1 NaOH solution and (f) D80 oil, where the samples were pre-dyed with trace amounts of methylene blue. The plot of t1mL vs pH of the emulsion (g). RESULTS AND DISCUSSION Reversible Switching of DUSNa-Stabilized Emulsion. The initial emulsion (Figure 1 a) consisted of mineral oil D80 and 20 mmol L−1 DUSNa solution (1:1, v/v). To avoid the hydrolysis of DUSNa, 0.1 mol L−1 NaOH aqueous solution was used as the solvent. After CO2 bubbling at room temperature, demulsification, i.e., phase separation, was observed (Figure 1 b). Dynamic light-scattering (DLS) results (Figure S3) indicated no detectable emulsified droplets in the separated oil phase, but we observed that a white and waxy suspended substance had formed in the separated aqueous phase (Figure 1 b). Outwardly, it looked like a phenomenon of flocculation. The suspended substance could be readily removed by filtration, and no detectable emulsified droplets or aggregates remained in the transparent aqueous filtrate (Figure S3). However, the separated phases (Figure 1 b) could be re-emulsified to obtain an emulsion again after N2 treatment (Figure 1 c) or NaOH basification (Figure 1 d). These results preliminarily confirmed that the DUSNa-stabilized emulsion could be reversibly switched between emulsion and phase-separated forms under the alternate stimulation of CO2 and N2 or NaOH. To determine the type of the emulsion, an initial emulsion was dyed with methylene blue, and then diluted with 0.1 mol L−1 aqueous NaOH solution or D80 oil, respectively. It was observed that the emulsion was easily diluted with the aqueous NaOH solution (Figure 1 e), but could not be diluted with the oil (Figure 1 f). Therefore, the emulsion was of the O/W type.12,18 The relative stability of the emulsion was measured as the time needed to separate 1 mL of H2O from 6 mL of emulsion (t1mL, min) at 30 °C. The t1mL for the initial emulsion was observed as about 17.5 min (Figure 1 g). Considering that 0.1 mol L−1 aqueous NaOH solution is of strong basicity


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(its pH is as high as 13), which may be hazardous in real applications, the relationship between t1mL and pH of the DUSNa-based O/W emulsion was determined (Figure 1 g). It was observed that t1mL was longer than 17 min at pH > 11, but shorter than 5 min at pH < 10; the relative stability of a DUSNa- stabilized O/W emulsion at pH 11 is almost the same as that at pH 13. Thus, there is no need for the initial DUSNa-stabilized O/W emulsion to be prepared at a high pH of 13.

Figure 2. Fluorescence microscope images and the number distribution of oil droplet diameters of the initial emulsion (a1 and a2), the N2-recovered emulsion (b1 and b2), and the NaOHrecovered emulsion (c1 and c2). The scale bar is 25 µm. For a quantitative assessment of the reversible process in Figure 1, the oil droplets in the initial (before CO2 bubbling, Figure 1 a) and the recovered (Figure 1 c and d) emulsions were observed


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using a fluorescence microscope (Figure 2 a1–c1). The emulsion samples in Figure 2 were predyed with a trace amount of Nile red. The number-averaged diameters (NAD) of oil droplets dispersed in the initial emulsion was around 9.6 ± 3.1 µm (Figure 2 a2), as compared to around 8.3 ± 3.0 µm for the N2-recovered emulsion (Figure 2 b2) and around 8.7 ± 3.4 µm for the NaOH-recovered emulsion (Figure 2 c2). Statistically, there were thus no obvious differences in NAD between the initial emulsion and the recovered emulsions. Moreover, when CO2 and N2 or NaOH were alternately applied, the O/W emulsion could be reversibly destabilized and regenerated over ten cycles without any significant variation. A mechanistic rationale for this is proposed in the following.


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Figure 3. The possible transformation between DUSNa and DUS (a), the reversible change in σ (b) at the interface of oil D80–20 mmol L−1 DUSNa aqueous solution. Mechanism of the Emulsion Switching. Generally, the capacity for emulsification (Figures 1 and 2) is governed by the molecular structures present in a solution of DUSNa or DUS. Additional understanding of the mechanism at the molecular level was thus sought from the perspectives of the transformation of the molecular structure and the corresponding interfacial activities of aqueous DUSNa or DUS triggered by CO2, N2 or NaOH. The dimethylamine group in DUSNa (Scheme 1) is a typical CO2-responsive group.11,14,21–25 Upon CO2 bubbling, the dimethylamine group could reasonably be protonated to form DUS by either Path 1 or Path 2 (Figure 3 a), lowering the pH of the initial DUSNa-based O/W emulsion from 13 to < 7 within 5 min (Figure S4). It was found that the interfacial tensions at the aqueous sample–air interface (γ, mN m−1) and aqueous sample–oil D80 interface (σ, mN m−1) (Figure S5) gradually decreased with increasing concentration of DUSNa, with break points (CMC) at about 1.0×10−2 and 9.5×10−3 mol L−1, respectively, comparable to that of sodium dodecyl sulfate (SDS, 8.1×10−3 mol L−1),18 a structurally related and traditional anionic surfactant. However, γ and σ for DUS are larger than those for DUSNa over the studied concentration range. The reduction of γ (∆γ ≈ 6.5 mN m−1) and σ (∆σ ≈ 13.8 mN m−1) for DUS are, in particular, less than 15 mN m−1 over the concentration range (Figure S3). The maximum differences in γ and σ between DUSNa and DUS at the CMC were as high as ∆γCMC ≈ 24.8 and ∆σCMC ≈ 23.5 mN m−1. Furthermore, the contact angle (θ, in degrees) of aqueous DUS solution (ca. 100.1°) on the surface of polytetrafluoroethylene is larger than that of DUSNa solution (ca. 80.8°) and close to that of H2O (ca. 107.6°, Figure S6). These results confirm that DUSNa is a typical surfactant, whereas DUS is a surface-active substance at best. DUSNa and DUS can be considered as the boundary states


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of the switching process (Scheme 1). Evidently, demulsification of the DUSNa-stabilized emulsion may be attributed to conversion of DUSNa to DUS by CO2 treatment, with loss of its ability to reduce σ. Compared to SDS, DUSNa is low-foaming (Figure S7) under an inert gas stream and the foam disappeared within 5 s when the gas stream was stopped. The foaming ability of DUS is almost the same as that of water. Unlike for previously reported CSASs,8,21–25 the poor foaming abilities of both DUSNa and DUS are beneficial for switching the DUSNastabilized emulsion back and forth, triggered by CO2 and N2, without the obvious disturbance of foam overflow.

Figure 4. σ at the interface of the fresh oil D80–H2O (■), the fresh oil D80–0.1 mol L−1 NaOH solution (▲), and the recovered oil D80–0.1 mol L−1 NaOH solution (▼). As shown in Figure 1 b, the solubility of DUS is poor in both water and D80 oil, but it becomes suspended in an aqueous phase, in contrast to CSASs reported in the literature, 8,21–25 which will facilitate its separation from the liquid matrix by filtration and its subsequent reuse. We could determine the fate of DUSNa after the phase separation from two aspects: i) the amount of DUSNa remaining in the separated D80 oil, and ii) the degree of conversion of


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DUSNa upon CO2 treatment. As shown in Figure 4, there was almost no DUSNa in the recovered oil, because σ at the interface of the recovered D80 oil–0.1 mol L−1 NaOH solution (ca. 38.3 mN m−1) was almost the same as those at the fresh D80 oil–0.1 mol L−1 NaOH solution (ca. 38.6 mN m−1) and the fresh D80 oil–H2O (ca. 38.4 mN m−1) interfaces. That is to say, the recovered oil could be considered as almost surfactant-free oil. The precipitate of DUS could be conveniently filtered by a 0.22 µm membrane filter. Around 2.3% of the DUSNa remained in the aqueous filtrate, while around 97.7% had been converted to DUS, as determined by titration with benzethonium chloride26 after the samples had been basified with NaOH. Generally, reduction of σ and mechanical, steric, and/or electrical barriers formed by the surfactant at the oil–water interface are thermodynamically and kinetically important stabilizing factors in surfactant-based emulsions.17,18 Thus, an almost complete conversion of DUSNa to DUS, an increase in σ (Figure S5), and loss of the DUSNa-based barrier at the water–oil interface lead to phase separation of the DUSNa-stabilized emulsion after CO2 treatment (Figure 1 b). Upon NaOH basification or N2 bubbling at 90 oC, the protonated dimethylamine group in DUS (Scheme 1 and Figure 3 a) is neutralized or deprotonated11,21–25 to regenerate DUSNa, whereby σ is restored from around 23.4 mN m−1 to the original value of about 4.4 mN m−1 (Figure 3 b). When CO2- and N2-bubbling or NaOH basification were alternately performed, σ could be reversibly changed and recovered over three cycles without any remarkable variation, which implies that a reversible transformation between DUSNa and DUS occurs smoothly (Scheme 1), resulting in reversible switching between phase separation and re-emulsification for the DUSNabased emulsion, as demonstrated in Figures 1 and 2. The reversible conversion of DUS to DUSNa could also be verified by changes in the TK of DUS and/or DUSNa (Figure S8) and the pH of the system (Figure S9). Obvious differences in TK and pH have been observed between


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N2-recovered and NaOH-recovered systems. These can be ascribed to the fact that the degree of conversion of DUS back to DUSNa for NaOH basification (around 99.4%) is larger than that for N2 treatment (around 65.2%). Interestingly, no obvious differences between the N2-recovered and NaOH-recovered emulsions were observed in terms of either the size of the dispersed oil droplets (Figure 2) or the relative stability of the emulsions (Figure S10). It seems that complete transformation of the surfactant is not needed for effective switching of the corresponding emulsion.8

Figure 5. Illustration of retrieving oil D80 and recycling DUSNa in SESW.


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Retrieving Oil D80 and Recycling DUSNa. In addition to facilitating emulsion breaking on demand, recovery and recycling of the surfactants and the oil resource are more valuable for the switchable-surfactant-enhanced soil washing in the remediation of oil-contaminated soil, especially oil accidentally leaked into soil. As illustrated in Figure 5, 95 g quartz sand, 5 g white oil, and 200 g DUSNa solution (20 mmol L−1, pH 13) were added to a vitreous SESW bath (Figure 5 a). After stirring for 2 h at room temperature, an emulsion (Figure 5 b) and the cleaned sand (Figure 5 c) were obtained by simple filtration. Upon CO2 treatment, phase separation was observed (Figure 5 d) to give the retrieved oil phase (Figure 5 e) and an aqueous phase (Figure 5 f). After treatment with N2 or NaOH basification, the recovered DUSNa solution was returned to the SESW bath for the next washing. The residual oil in the cleaned sand (Figure 5 c) was around 3.2 % of the total oil, around 92.1% of the total oil was retrieved, and around 95.2% of the DUSNa remained in the aqueous phase. The recovered DUSNa was recycled twice, the residual oil remained was around 2.8 % of the total oil, and the oil retrieval was around 93.4% to 94.1%. However, a slight decrease in the DUSNa remaining in the aqueous phase was observed from 95.2% to 90.8% after twofold recycling. When the aqueous phase (Figure 5 f) was filtered through a 0.22 µm membrane filter, the values of TOC and COD of the aqueous filtrate were estimated to be around 402.2 and 634.7 mg L−1, respectively, remarkably smaller than those for a control sample of SDS (the TOC and COD for which were as high as 3276.8 and 6778.4 mg L−1, respectively). However, after a simple and conventional treatment with active carbon and ionexchange resin, the TOC and COD values of the filtrate were remarkably reduced to 19.1 and 35.6 mg L−1, respectively, implying that the treatment process of the waste water generated from the DUSNa-based SESW (Figure 5) is simple and highly efficient.


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In summary, we have demonstrated that by incorporating a CO2-switchable tertiary amine group at the terminal position of the hydrophobic tail in a traditional anionic surfactant, a novel CO2-switchable anionic surfactant of DUSNa is formed. Upon CO2 bubbling, the tertiary amine group in DUSNa is protonated, leading to DUS; upon N2 bubbling at 90 oC or basification with NaOH, DUS is converted back to DUSNa. Consequently, a DUSNa-stabilized O/W-type emulsion could be reversibly switched between phase separation and re-emulsification triggered by CO2 and N2 bubbling at 90 oC or basification with NaOH. The remarkable versatility and simplicity of the system may benefit many technological applications. For example, the present DUSNa-stabilized O/W emulsion may be used to efficiently retrieve oil from oil-polluted sand, and then to enrich the oil through complete oil/water separation induced by simply bubbling CO2 without obvious foam overflow. In particular, DUS is insoluble in both water and ordinary hydrocarbon oil, but becomes suspended in the aqueous phase. Subsequently, the recovered oil is free from contamination by DUSNa or DUS, and the aqueous suspension of DUS may be recovered and subsequently recycled. After filtering off the insoluble DUS followed by a simple and conventional treatment with active carbon and ion-exchange resin, both the TOC and COD of the filtrate were remarkably reduced to less than 40 mg L−1. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Details on Materials and Methods (Scheme S1 and Chart S1), additional results of Figures S1–S10 (PDF).


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Xuefeng Liu: 0000-0002-9506-7798 Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (grant No. 21673103). We are also grateful to Prof. Guanjun Tao (State Key Laboratory of Food Science & Technology, Jiangnan University) for the ESI MS measurements and discussion. REFERENCES (1) Trellu, C.; Mousset, E.; Pechaud, Y.; Huguenot, D.; van Hullebusch, E. D.; Esposito, G.; Oturan, M. A. Removal of hydrophobic organic pollutants from soil washing/flushing solutions: a critical review. J. Hazard. Mater. 2016, 306, 149–174. (2) Gitipour, S.; Hedayati, M.; Madadian, E. Soil washing for reduction of aromatic and aliphatic contaminants in soil. Clean: Soil, Air, Water 2015, 43, 1419–1425. (3) Mao, X.; Jiang, R.; Xiao, W.; Yu, J. Use of surfactants for the remediation of contaminated soils: a review. J. Hazard. Mater. 2015, 285, 419–435.


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TOC

Anionic surfactant and its emulsion reversibly response to CO2 allows subsequent retrieval oil and recycling surfactant simultaneously.


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Retrieving Oil and Recycling Surfactant in ... - ACS Publications

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